A communication apparatus for performing wireless or wired information communication includes various signal processing devices such as an amplifier, mixer, and filter, A bandpass filter as a kind of filter passes a signal having a frequency falling within a passband, and cuts off (attenuates) a signal having a frequency falling outside the passband (falling within a stopband). For example, the characteristics of a bandpass filter such as the center frequency of the passband and the passband width are designed in accordance with the specifications of a communication system to which the bandpass filter is applied.
For example, as the skirt characteristic (i.e., the cutoff characteristic near the boundary between the passband and stopband) of a bandpass filter becomes steeper, a necessary guard band width decreases, so the frequency utilization efficiency can be increased. The bandpass filter can be installed by a planar circuit filter such as a microstrip line filter. The planar circuit filter can achieve a steep skirt characteristic by cascading resonators.
When forming resonators by using a general conductor material, an increase in transmission loss caused by cascading the resonators poses a problem. For example, even when resonators are formed by using an electrically good conductor material such as copper (Cu) or silver (Ag), the number of resonators which can be cascaded is limited. To cascade a number of resonators, it is effective to form the resonators by using a superconducting material. Even when compared to the electrically good conductor material, the superconducting material has a very low surface resistance in a high-frequency region, and can suppress an increase in transmission loss caused by cascading. That is, a bandpass filter having a steep skirt characteristic can be implemented by cascading resonators using the superconducting material.
Also, a tunable filter having a variable filter characteristic (i.e., a variable frequency characteristic) is sometimes necessary to, e.g., make the use band changeable. Some kind of tunable filters have a variable passband center frequency, and the passband and stopband also change in accordance with a change in center frequency. When the center frequency of the passband is changed, deviation of the shape of the filter characteristic from an ideal shape is unpreferable. Accordingly, the tunable filter is also required to suppress the disturbance of the shape of the filter characteristic when the center frequency of the passband is changed.
General required specifications of the tunable filter are that the variable range of the band is wide, the band change is continuous, the insertion loss is small, the cutoff characteristic is steep (i.e., the Q value is high), the whole filter including an adjusting mechanism is light and small, and the operation reliability and reproducibility are high.
The filter characteristics of the bandpass filter such as the center frequency of the passband, the passband width, the cutoff characteristic, and the out-of-band suppression characteristic are determined by, e.g., the resonance frequency of each resonator, the coupling coefficient between resonators, and the external Q value. Accordingly, a tunable filter can be implemented by changing the resonance frequency of each resonator and the coupling coefficient between resonators. That is, when the principle of the tunable filter is explained by the materials science, the tunable filler changes the filter characteristics by changing at least one of the effective relative dielectric constant and effective relative permeability by some means. On the other hand, when the principle of the tunable filter is explained by the circuit science, the tunable filter changes the filter characteristics by changing at least one of the capacitance and inductance by some means.
Presently, methods of implementing the tunable filter are roughly classified into three categories, i.e., electric field control, magnetic field control, and mechanical control. To maintain a high Q value, an adjusting mechanism having a small loss is favorable. The feature of a mechanical-control-type tunable filter is that the variable range of the band is maximum and the loss is small. This mechanical-control-type tunable filter changes the filter characteristics by changing the gap length between a (superconducting) filter substrate and a characteristic tuning member (typically, a dielectric material or magnetic material).
Before an operation is started, the initial gap length of the mechanical-control-type tunable filter is sometimes adjusted in order to finely correct a shift of the filter characteristics caused by, e.g., manufacturing variations. Although the initial gap length is manually adjustable, manual adjustment is unpreferable from the viewpoints of speed and accuracy. Especially when the tunable filter is a superconducting filter, it is necessary to cancel/set a vacuum low-temperature environment whenever performing an initial gap length adjusting work and filter characteristic checking work. Therefore, it takes a long time to complete these works.